1. Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly to a multi-wavelength semiconductor laser device capable of reducing operation current of a low-output semiconductor laser diode while providing a sufficient resonant length to a high-output semiconductor laser diode.
2. Description of the Related Art
A general semiconductor laser device comprises p-type and n-type clad layers for current injection, and an active layer disposed between the clad layers in which induced emission of photons substantially occurs. Since an upper clad layer of the general semiconductor laser device is formed into a ridge structure, current injection efficiency is improved and at the same time the ridge structure serves as a waveguide for the light emitted from the active layer.
With the recent distribution of CD-RWs and DVD-RWs, multi-wavelength semiconductor laser devices capable of oscillating laser light of two or more different wavelengths have been required in the art. Particularly, multi-wavelength semiconductor laser devices, including two-wavelength semiconductor laser devices, are increasingly employed as light sources for operating both CD players having a relatively low data density and DVD players having a relatively high data density. For example, a multi-wavelength semiconductor laser device is produced by integrating a semiconductor laser diode (hereinafter, referred to simply as an “LD”) emitting light at 780 nm and a semiconductor LD emitting light at 650 nm on a single substrate. In such multi-wavelength semiconductor laser devices, maximum outputs of laser light at each wavelength are different.
According to a conventional multi-wavelength semiconductor laser device, since waveguides for light at each wavelength have a rectilinear structure (that is, a straight structure), the resonant length of an LD requiring a low output is identical to that of an LD requiring a high output. Accordingly, there is a problem in that an increase in the operation current of the LD requiring a low output is inevitable.
FIGS. 1a and 1b are a cross-sectional view and a plan view of a conventional multi-wavelength semiconductor laser device, respectively.
Referring to FIGS. 1a and 1b, the conventional multi-wavelength (two-wavelength herein) semiconductor laser device comprises a first LD A and a second LD B formed on one substrate 11. The first LD A and the second LD B are electrically and optically separated from each other by a device separation region I, and are configured in such a manner that they emit light at first and second wavelengths, which have different outputs. For example, the first LD is made of an AlGaInP-based semiconductor and emits high-output laser light at 650 nm, while the second LD is made of an AlGaAs-based semiconductor and emits low-output laser light at 780 nm.
The first LD A includes a first conductivity-type clad layer 12a, an active layer 13a, a second conductivity-type upper clad layer 14a, and an etch stop layer 15a sequentially formed on the substrate 11. Like the first LD A, the second LD B includes a first conductivity-type clad layer 12b, an active layer 13b, a second conductivity-type upper clad layer 14b, and an etch stop layer 15b sequentially formed on the substrate 11. In addition, first and second ridge structures 30a and 30b are formed on the respective laminates. The first ridge structure 30a includes a second conductivity-type upper clad layer 16a, a second conductivity-type cap layer 17a, and a second conductivity-type contact layer 18a formed on the etch stop layer 15a. A current blocking layer 21a for blocking current dispersion is formed around the first ridge structure 30a. Likewise, the second ridge structure 30b includes a second conductivity-type upper clad layer 16b, a second conductivity-type cap layer 17b, and a second conductivity-type contact layer 18b formed on the etch stop layer 15b. A current blocking layer 21b for blocking current dispersion is formed around the second ridge structure 30b. 
As shown in FIG. 1b, the ridge structures 30a and 30b, serving as waveguides for laser light, of the respective first and second LDs A and B have a rectilinear structure. Accordingly, the first and second LDs A and B have the same resonant length L. At this time, unfavorable phenomena, such as gain saturation and catastrophic optical damage (COD), may take place in the high-output LD due to increased current density. To prevent these phenomena, the high-output LD (i.e. the first LD) is required to have a large resonant length of at least 600 μm. However, such a large resonant length L of the first LD A is applied to the low-output LD (i.e. the second LD B). The unnecessarily large resonant length of the low-output LD B causes a higher operation current than required to operate the low-output LD.
Further, when the resonant length L is increased in order to prevent the occurrence of COD in the high-output LD, the number of multi-wavelength semiconductor laser devices capable of being produced on a single wafer is decreased. Taking into consideration these problems, a process wherein the width W of a semiconductor laser device is reduced is currently employed. However, since the length L remains unchanged despite the small width of the semiconductor laser device, the productivity is not greatly improved. Moreover, process margins are worsened in subsequent steps and mechanical damage tends to occur due to the small width W.